Technical Field
The present invention relates to a honeycomb filter.
Background Art
Particulates (hereinafter also referred to as PMs or soot) such as soot in exhaust gas discharged from internal combustion engines including diesel engines cause damage to environment and human bodies, and these days people have paid attention to this problem. Since exhaust gas contains toxic gas components such as CO, HC, and NOx, people also worry about the influences of the toxic gas components on the environment and human bodies.
To overcome this problem, various filters having honeycomb structures (honeycomb filters) formed of porous ceramics such as cordierite and silicon carbide have been proposed as exhaust gas purifying apparatus. Such honeycomb filters are connected to internal combustion engines to capture PMs in exhaust gas, or to convert the toxic gas components such as CO, HC, and NOx in the exhaust gas into nontoxic gas.
In addition to CO in the exhaust gas, it is also necessary to purify CO generated during combustion of PMs captured in the honeycomb filter. JP 2008-284542 A discloses a honeycomb filter which supports an oxidation catalyst on a predetermined area for efficiently converting CO.
For enhancing the fuel economy of internal combustion engines and avoiding troubles derived from an increase in the pressure loss during operation, various honeycomb filters have been proposed including those in which the initial pressure loss is lowered by improvement of the cell structure and those in which the rate of increase in the pressure loss is low when a certain amount of PM is accumulated.
JP 2003-284542 A, WO 2004/024294, U.S. Pat. No. 4,417,908, and WO 2007/134897 disclose such filters.
JP 2008-284542 A discloses a honeycomb filter including: an outlet-side catalyst supporting area formed from an end face on a gas outlet side toward an end face on a gas inlet side; an inlet-side catalyst supporting area formed from an end face on the gas inlet side toward an end face on the gas outlet side; and a catalyst unsupporting area with no catalyst supporting layer, the catalyst unsupporting area being between the outlet-side catalyst supporting area and the inlet-side catalyst supporting area. The catalyst unsupporting area has a higher thermal conductivity than the outlet-side catalyst supporting area and the inlet-side catalyst supporting area. Ratios or the length of the outlet-side catalyst supporting area, the length of the inlet-side catalyst supporting area, and the length of the catalyst unsupporting area, relative to the entire length or the honeycomb filter in the longitudinal direction satisfy a predetermined relationship.
FIG. 19A is a perspective view schematically illustrating a honeycomb filter disclosed in WO 2004/024294. FIG. 19B is a perspective view schematically illustrating a honeycomb fired body forming the honeycomb filter.
As shown in FIGS. 19A and 19B, WO 2004/024294 discloses a honeycomb filter 90 that includes a plurality of honeycomb fired bodies 100 combined with one another with adhesive layers 105 residing therebetween, and an periphery coat layer 106 formed on the periphery of the combined honeycomb fired bodies, wherein the honeycomb fired bodies 100 each include exhaust gas introduction cells 102 each having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side, and exhaust gas emission cells 101 each having an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side; the exhaust gas emission cells 101 each have a square cross section perpendicular to the longitudinal direction of the cells; the exhaust gas introduction cells 102 each have an octagonal cross section perpendicular to the longitudinal direction of the cells; and the exhaust gas emission cells 101 and the exhaust gas introduction cells 102 are alternately (in a grid-like pattern) arranged.
Hereinafter, in the explanation of the embodiments of the present invention and background arts, a cell having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side is simply described as an exhaust gas emission cell. Moreover, a cell having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side is simply described as an exhaust gas introduction cell, a first exhaust gas introduction cell, or a second exhaust gas introduction cell.
The term just described as “cell” means both of an exhaust gas emission cell and an exhaust gas introduction cell.
Moreover, cross sections perpendicular to the longitudinal direction of cells including exhaust gas introduction cells, exhaust gas emission cells, or the like are simply described as cross sections of the exhaust gas introduction cells, exhaust gas emission cells, or the like.
FIG. 20A is a perspective view schematically illustrating a honeycomb filter disclosed in U.S. Pat. No. 4,417,908. FIG. 20B is a view schematically illustrating an end face of the honeycomb filter.
U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 in which all cells have the same square cross-sectional shape as shown in FIGS. 20A and 20B. In the honeycomb filter 110, exhaust gas emission cells 111 each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side are adjacently surrounded fully by exhaust gas introduction cells 112 and 114 each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side across cell walls 113. In the cross section, a side of the exhaust gas introduction cell 112 faces the exhaust gas emission cell ill across the cell wall 113, whereas the corners of the exhaust gas introduction cells 114 respectively face the corners of the exhaust gas emission cells 111. Thus, none of the sides forming the cross sections of the exhaust, gas introduction cells 114 faces the exhaust gas emission cells 111.
WO 2007/134897 discloses a honeycomb filter including exhaust gas introduction cells and exhaust gas emission cells, wherein, an exhaust gas emission cell having a hexagonal cross section is surrounded by six exhaust gas introduction cells each having a hexagonal cross section, and the cross-sectional areas of all the exhaust gas introduction cells are larger than the cross-sectional areas of all the exhaust gas emission cells.